1. Field
The present disclosure generally relates to the design of multi-chip modules (MCMs). More specifically, the present disclosure relates to an MCM that couples an optical signal between optical waveguides on different substrates using an optical interposer.
2. Related Art
Optical signaling based on silicon photonics has the potential to alleviate off-chip bandwidth bottlenecks, as well as to provide low latency chip-to-chip communication. Interconnects with these capabilities can facilitate new system architectures that include multiple chips, with multi-threaded cores. For maximal density, the physical package for such a system may employ a combination of planar packaging and vertical chip stacking as needed. An example of such a system is a multi-chip module (MCM) or ‘macrochip’ that includes a logically contiguous piece of photonically interconnected silicon that integrates processors, memory and a system-wide interconnect.
In the macrochip, optical couplers, such as optical proximity couplers (OP×Cs), couple the distributed processors to optical routing layers, which support low-latency, wavelength-division multiplexed (WDM) optical links between chips using silicon-on-insulator (SOI) optical waveguides. These optical waveguides form an interconnect network that provides low-power, high-bandwidth, and high-density communication between the chips in the macrochip. Moreover, each of the chips in the macrochip can be interconnected to every other chip via the WDM optical links that run in orthogonal directions on two routing layers. The optical signals from the chips are coupled into, and between, the routing layers using face-to-face OP×Cs.
However, achieving high-fidelity signaling across a multi-chip geometry, such as the macrochip, with low-loss coupling and broadband transmission is a major challenge. Many of the existing techniques used to implement inter-layer OP×Cs (such as mirror-reflecting couplers, diffraction-grating couplers, butt-coupled optical waveguides and lens couplers) typically have insertion losses between 2.8 and 4.5 dB per OP×C hop. These loss numbers are very high and can be attributed to: alignment errors between the top and bottom OP×C surfaces, light clipping at each mirror surface, back scattering, grating-etch errors, mode mismatch, etc. In a system employing a few such OP×C hops per channel, these high losses can severely impact the optical-link budget, which can significantly increase the performance requirements (and, thus, the cost) of other components in the system, such as the optical sources and/or the receivers. As a consequence, it can be difficult to obtain high-fidelity signaling in such a multi-chip geometry with low-loss coupling and broadband transmission, which can adversely impact the performance of the macrochip.
Hence, what is needed is an MCM without the problems described above.